INTRODUCTION
Acute bronchiolitis is a major cause of morbidity and mortality in young children associated with steep healthcare costs worldwide1. The clinical course of bronchiolitis ranges from mild lower respiratory tract symptoms to severe respiratory distress from lower airway obstruction. Although there are no universally accepted criteria to predict disease progression, decisions regarding further evaluation and management are based on clinical judgement and the presence of known risk factors2.
Antimicrobial peptides (AMPs), also known as host defence peptides, are short and generally positively charged oligopeptides expressed in a wide variety of human tissues. AMPs are secreted into the biofilm covering the inner surface of the airways and may be involved in the direct inactivation of viruses. Indeed, recent studies support their antiviral activity3 and consequently their therapeutic potential in novel antiviral strategies4 against viruses for whom no effective treatment is yet available such as SARS-CoV-25.
Among over one hundred human antimicrobial peptides recognized, defensins and human cathelicidin are the most extensively studied. Human cathelicidin is a 18 kDa cationic antimicrobial protein called hCAP-18, which is encoded by the CAMP gene and is cleaved into two peptide chains, FALL-39 and LL-37. Recent data show that β-defensin-2 and LL-37 may have in vitro antiviral activity against Respiratory Syncytial Virus (RSV)6, as well as to other viruses implicated in the pathogenesis of bronchiolitis7,8. Interestingly, the exogenous application of LL-37 had protective effect in vivo in murine models against RSV and Influenza A pulmonary infections9,10. In addition, recent laboratory studies have shown that vitamin D plays a vital role in up-regulating the expression of AMPs in the sight of infection, suppressing inflammation by modulating cytokine production, and, thereby, reducing the severity of viral lower respiratory tract infections11,12.
Despite the lack of previous relevant studies, these data suggest that higher airway mucosal concentrations of AMPs may correlate positively with serum vitamin D levels and a mild disease course. Therefore, the aim of this study is to examine the association between the mucosal production of LL-37 and β-defensin-2 in relation to serum vitamin D levels and the severity of bronchiolitis.